1. Field of the Invention
The invention relates to electrophotography and, more particularly, to automatically controlling the amount of electrical energy supplied to a fuser.
2. Description of the Prior Art
In an electrophotographic apparatus, for example a xerographic copier, a pattern of toner is placed on paper in accordance with an image on an original document. The toner is fixed on the paper to form a permanent copy of the original image by applying a combination of heat and pressure to the toner and paper. For example, in a typical copier, the paper carrying loosely adhering toner is passed between a heated fuser roll and a backup roll to essentially melt the toner into the paper.
The quality of the result depends upon the temperature to which the paper is brought during the fusing operation. At the extremes, too high a temperature will undesirably remove toner from the paper, while too low a temperature will fail to properly bond the toner to the paper. In practical systems, a major problem is non-uniformity of heat application over the duration of a "copy run"; the copying of one or more originals during a single continuous operator-initiated job. Successive sheets of paper may reach different temperatures, not all within the acceptable range, and different sizes of paper may reach different temperatures at different times. Further, within a copy run, not all parts of the same sheet of paper will be within the acceptable range of temperature during the entire fusing operation, resulting in a wide spectrum of quality problems involving the paper and toner. These problems are believed to be due to many factors, such as: differing thermal load presented by different qualities, sizes and weights of paper; materials and environmental factors affecting the thermal transfer between the fuser surface and the paper; and the degree of temperature control achieved at the surface where the paper contacts the fuser. It is the latter factor that is addressed by this invention.
A physically massive fuser roll will eventually reach a stable surface temperature which is substantially independent of external factors such as the size and quantity of paper placed in contact with it or the rate at which the paper is fed past the roller. However, copier size, energy availability and time-to-first-copy restrictions dictate that a fuser roll be as physically small as possible. Commercial copiers acceptable in an office environment ideally must be physically small, cannot generate excessive quantities of heat and must be connected to a conventional wall outlet. The fuser heater is often a major source of heat and user of electrical energy, and the size of the fuser is a significant factor in the size of the copier. Therefore, a small fuser is desirable because it generates less heat, uses less power, takes up less space, etc. However, the smaller the fuser roll mass relative to the mass of the copy paper, the greater will be the thermal load effect of the paper on the roll. That is, each sheet will have a greater cooling effect on a small roll than on a large roll. The techniques used to maintain the fuser roll at a constant temperature, regardless of external thermal factors, thus become extremely important for a fuser roll which has a mass which is not very much greater than the mass of the copy paper. These are especially important where a large thermal load is present because (as in a fast copier) many sheets of paper are rapidly fed through the fuser.
The prior art discloses solutions to the problem of accurately controlling fuser temperature. The simplest prior art technique provides an on-off thermostat in series with the fuser heater. While this approach to the problem controls fuser temperature, on-off cycling does not always provide the accuracy required by compact and fast copiers, even where the thermostat is extremely accurate. The primary disadvantage of on-off cycling is that the peak power requirements are significantly greater than the average power used for heating. Thus, a copier with an on-off thermostat must be connected to an undesirably large current source. For example, a typical commercial copier requires a dedicated power circuit capable of supplying as much as 3-5 kilowatts. This large power requirement is determined in part by "worst case" fuser conditions, such as the age of the heating element, original manufacturing tolerances and line voltage variations.
Another prior art solution is to supply only portions of each available power cycle to heat the fuser. The portions are selected as a function of the fuser temperature sensed by a thermostat. One shortcoming of this approach is that since it requires electronic switching of substantial load currents, physically large electrical parts are required, energy may be wasted and undesirable electromagnetic radiation occurs.
In U.S. Pat. No. 3,961,236, accuracy is increased by monitoring fuser heater voltage and current and then adjusting input power to maintain power consumption constant. Adjustment is obtained by controlling the number of complete alternating current half-cycles supplied by a power source to the fuser. Since the load current is supplied in complete half-cycles, switching of the load current may be accomplished at zero-crossings to greatly reduce component size, electromagnetic noise and, in some cases, energy dissipation. U.S. Pat. No. 3,541,429 discloses a welding current phase angle controller which selects portions of alternating current half-cycles in accordance with preselected digital numbers. A lamp dimmer disclosed in U.S. Pat. No. 3,691,404 selects groups of alternating current cycles in accordance with digital quantities chosen by manually adjusting a switch. The cycles are switched at zero-crossings. U.S. Pat. No. 3,259,825 shows a servo drive wherein varying numbers of positive half-cycles are selected at zero-crossings as a function of a control voltage. Power to a heat source is controlled in U.S. Pat. No. 3,878,358 by removing full cycles from a succession of alternating current cycles, as a function of the output from a heat sensor. U.S. Pat. No. 3,456,095 selects the frequency at which positive half-cycles of power are supplied to a heater as a function of signals from a heat sensor.
While the foregoing prior art addresses the problem of accurately controlling the temperature, several shortcomings remain. A significant shortcoming of most prior art designs is the large peak power requirement discussed with reference to on-off thermostats, but also applicable to other prior art solutions. For example, depending upon which approach is selected, high, instantaneous or average currents are switched, special electric supply sources are required, electromagnetic noise is generated, fuser temperature is not sensed, undesirable direct current components are present (when the difference between the numbers of positive and negative half-cycles is not zero), large physical size is necessary, finely graduated adjustments are not possible, manual intervention is required, etc. Some of these shortcomings are discussed in a contemporaneous article by McCarthy and Danesh entitled "A Novel Method of Direct Digital Integral-Cycle Power Control" published May, 1978, in the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS AND CONTROL INSTRUMENTATION at pages 149-154. The authors disclose a method for controlling power by subdividing half-cycles into pulse trains having frequencies which are binary submultiples of the power supply frequency. The half-cycles begin and end at zero-crossings and, if controlled in pairs, minimize the load current direct current component. In an application, Ser. No. 921,659, filed July 3, 1978, "Microprocessor Controlled Power Supply for Xerographic Fusing Apparatus", by L. M. Ernst, assigned to International Business Machines Corporation, a copier fuser receives a first number of half-cycles per unit of time in one mode and a different number in another mode.